I've used Intellicad since it was launched by VISIO (Intellicad 98, if memory serves)
Just upgraded my CMS version to version 10. ($89 or £69 here in the UK).
I'm still running 9 atm as the upgrades are effectively a NEW install, which is a pain, and I'm putting that off.
Yes it does not have ALL the functionality of Autocad, but for the price........
I don't use 3rd party ad-ons, so can't comment there.
I have found that customisation is just the same as in Autocad, I can load my old mnu files.
It will depend on what you want from it, is it 2D or3D
here are a couple of images of work I have done in Icad. (note: images were saved from Detect3D
S.